dispatch: install the correct modulemap when building static

There is a subtle difference between libdispatch built dynamically and
statically: DispatchStubs.  We erroneously emit ObjC runtime calls into
the build and the stubs provides a shim to provide a definition on
non-ObjC runtime enabled targets.  When built dynamically, this is
internalised and distributed as part of dispatch.dll/libdispatch.so,
however when built statically, the consumer is responsible for linking
against DispatchStubs.  The modulemap reflects this and we need to
ensure that we correctly provide that modulemap.

Thanks to @MaxDesiatov for the help with debugging and testing a fix for
this!

Fixes: rdar://23335318

Author: compnerd

dispatch/CMakeLists.txt

@@ -1,8 +1,10 @@
 
 if(CMAKE_SYSTEM_NAME STREQUAL Darwin)
   set(DISPATCH_MODULE_MAP ${PROJECT_SOURCE_DIR}/dispatch/darwin/module.modulemap)
-else()
+elseif(BUILD_SHARED_LIBS)
   set(DISPATCH_MODULE_MAP ${PROJECT_SOURCE_DIR}/dispatch/generic/module.modulemap)
+else()
+  set(DISPATCH_MODULE_MAP ${PROJECT_SOURCE_DIR}/dispatch/generic_static/module.modulemap)
 endif()
 configure_file(dispatch-vfs.yaml.in
   ${CMAKE_BINARY_DIR}/dispatch-vfs-overlay.yaml